A couple of things that you should keep in mind is that most people are mixing up the two Atom models:
Silverthorne - MID's only - 13mm x 14mm package - single chip Paulsbo chipset
Diamondville - mini-ITX & Notebooks (4W at 1.6Ghz) - 22mm x 22mm package - 945GC Chipset (20W)
Via Nano - Flexible design chose - 21mm x 21mm package - single chip VX800 (5W)
If you would compare platforms:
L2200 + VX800 = 22W total max
N230 + 945GC + ICH7 = 24W total max
Via also allows much more flexibility in the design (no restrictions).
At present, Intel states that manufacturers cannot use its Atom processors in portable systems with a screen size of over 10 inches. Intel hopes that such a restriction will prevent Atom from infringing on its Core 2 mobile market.
Intel also only allows Atom processors to be used with mini-ITX motherboards which feature no PCI Express expansion slots and include only a single DIMM slot.
Even my platform comparison isn't fair since a L2200 will be much faster than a N230.
An U2400 (1.3Ghz) will be fast enough and that one is rated 8W (13W for platform).
You should keep in mind that Diamondville has Hyperthreading and because of that, it's result in synthetic SMT-aware benchmarks (like CrystalMark) are inflated. Real life performance will be lower because in real life Hyperthreading is less efficient.